A touch screen offers a simple, intuitive interface to a computer or other data processing device. Rather than using a keyboard to type in data, a user can transfer information through a touch screen by touching an icon or by writing or drawing on a screen. Touch screens are used in a variety of information processing applications. Transparent touch screens are particularly useful for applications such as cellphones, personal data assistants (PDAs), and handheld or laptop computers.
Various methods have been used to determine touch location, including capacitive, resistive, acoustic and infrared techniques. Touch location may also be determined by sensing the force of the touch through force sensors coupled to a touch surface. Touch screens that operate by sensing touch force have several advantages over other technologies mentioned above. First, force sensors do not require the touch surface to be composed of special materials that may inhibit optical transmission through the touch surface, as in a resistive touch sensor. Further, force sensors do not rely on a lossy electrical connection to ground, as is required by a capacitive touch screen, and can be operated by a finger touch, gloved hand, fingernail or other nonconductive touch instrument. Unlike surface acoustic wave technology, force sensors are relatively immune to accumulations of dirt, dust, or liquids on the touch surface. Finally, a force sensor is less likely to detect a close encounter with the touch surface as an actual touch, which is a common problem with infrared touch screens.
A force based touch screen may be built with a minimum of three force sensors spaced in a triangular pattern under a touch surface. Such an arrangement may provide signals sufficient to determine the net perpendicular force and the two moments necessary to compute touch location. Touch screen devices also may be built with a larger number of sensors. Commonly, four corner sensors may be used, in part to harmonize with the symmetry of the rectangular touch surface typically required. Upon application of a touch, the forces sensed by the touch screen sensors may be used to determine the touch location. However, determination of the touch location may be affected by a number of factors in addition to the touch force. Twisting, squeezing or otherwise distorting the touch screen during a touch may cause inaccuracies in the touch location determination.